As far as I know, the reasoning is that the current theme controls all the aesthetics, including colors and fonts. This is supposed to give you a coherent visual experience. I still thinks it's bollocks, but that's the explanation. On the other hand, themes are really easy to tweak. Still, it's bollocks.

From what I have read in mailing list threads, the Gnome team want to ensure that when someone looks at your desktop, they know its Gnome. So they don't want you mucking with the defaults and changing how it looks. A reason why you basically can't change the top black bar.

Without a simple working sollution like the pixbuf engine for gtk2 all themes for gnome will keep looking like shite. Flat, dull and css crapness.
In the mailing list you can read why, they want to kill off all 3d party additions to the gnome desktop.